Corrosion of metals in contact with acids is caused by reaction between them. In many industrial processes and equipment (e.g., piping, tanks, valves, cooling towers, heat exchangers, etc), acids (or aqueous acidic solutions, collectively called “acids”) are used for periodic cleaning of metallic components (e.g., removing of deposited scale); and such exposure to acids causes enhanced corrosion of the metals. Sometimes corrosion inhibitors are also added to water to reduce corrosion caused by its reaction with metals. In some industrial applications such as oil wells, one may use strongly corrosive acids for cleaning well bores, particularly for newly installed wells and also for periodic stimulation of existing wells to restore production rates of oil and gas. Acidization or acid treatments of oil and gas wells is also done as part of fracking process, where typically the acidization treatment is followed by injecting large volume of water and sand particles with other components under pressure. This stimulation is done to dissolve debris-blocking porosity and cracks in rock formations which block the flow of oil or gas. Since the addition of these acids in the wells is done through metallic (mainly ferrous) pipes, the acids can corrode them. In the petroleum industry, corrosion problems (reaction between acid and metals) intensify with depth of the wells, as the temperature increases with depth. In some cases, acids can come in contact metals at elevated temperatures in a range from about 60° C. to about 250° C. under high pressure and rapid flow conditions. Although the protection of ferrous metals is an important focus; the present invention may also be used in protecting other metals and alloys from corrosion, especially acid corrosion. Since corrosion accelerates under elevated temperatures and in the presence of strongly corrosive acids, corrosion inhibition of metals under these conditions becomes more important.
The corrosion inhibition being addressed here is different from the corrosion caused or increased by microbes (such as sulfate reducing anaerobic bacteria), where for example iron may be converted to soft iron sulfide. Such corrosion protection is achieved by killing the bacteria and or protecting metals from the gases released by such bacteria. In this invention the main issue being addressed is acid caused corrosion rather than microbially-induced corrosion (MIC). The purpose of the present invention is to reduce corrosion by preventing reaction between acids and metals by incorporating additives of the present invention in the acidic fluids. Strongly corrosive acids are generally pumped down oil wells in so called acidification treatments which are the focus of the present invention, while the biocides used to kill microbes are usually incorporated in aqueous solutions without such acids.
The extent of corrosion is typically expressed in terms of the weight loss/area (as kilograms of reduction in metal weight due to corrosion for each square meter of exposed area or pounds/sq ft, etc.) in a specified period of time. In some cases corrosion is also expressed in terms of reduction in the number of corrosion pits (when pitting corrosion takes place). When corrosion is measured on samples of identical geometry then it may also be expressed as % weight loss for relative comparison. The focus of this disclosure is on additives for aqueous acidic solutions so that corrosion of a metallic component is decreased when they are put in contact with acids. Typically higher concentrations of the corrosion protection agent will be required to achieve a desired level of corrosion inhibition with increasing acid strength and temperature. In some cases, the corrosion inhibitors of this invention may also be added to other petroleum well completion and production fluids.
Important processing steps in the petroleum industry where acids are typically added include:                1. Drilling, completion and workover fluids.        2. Cleaning of well bores (e.g., newly cemented wells)        3. Hydraulic fracturing (fracking) process.        4. Flooding and injecting of water during production of oil and gas.        5. Pipelines, tank flush, pipeline pigging and scraping and packer fluids (maintenance).        6. Well stimulationAmong these processing operations, strong acids are commonly used for cleaning well bores, fracking and well stimulation. The strong acids dissolve cement residues from well bores and in fracking and stimulation they dissolve constituents of underground formations to increase the porosity of these formations in order to enhance oil flow and recovery.        
Some of the ferrous metals used in the petroleum industry for which corrosion protection is desired are chrome steels, low carbon steels, duplex steels, stainless steels, martensitic alloy steels, ferritic alloy steels, austenitic stainless steels, precipitation-hardened stainless steels, high nickel content steels, etc. Some of the specific alloys routinely used in the petroleum industry for tubing and piping applications include N-80, L-80, J-55 P-110, 13Cr (regular, modified and super-chrome), 22Cr, QT800, QT900 and QT 1000, etc. To protect the in-place tubes cemented to the well bores and to reduce the amount of acid needed, one lowers a flexible tubing (coiled in a spool, and called coil tubing) into the well bore close to the bottom so that acid can be delivered through this tubing. These coil tubings are typically made of low carbon steel and may corrode with repeated acid use. Such tubes also need protection from the acids to prolong their lives. Some examples of typical acidic compositions used in the petroleum industry are:                1. Hydrochloric acid in a range of 5 to 34% strength by weight.        2. Acetic acid in a range of 1 to 15% strength by weight.        3. Formic acid in a range of 1 to 10% strength by weight.        4. Hydrofluoric acid in a range of 0.5 to 6% strength by weight.        5. Mixtures of these acids in such concentrations.        
It is well known that hydrochloric acid, acetic acid and formic acid are widely used in carbonate formations, while mixtures of hydrochloric acid and hydrofluoric acid are used in sandstone formations (e.g., 3% HF+12% HCl). For purposes of the present application, acids such as those listed in 1-5 are referred to as strongly corrosive acids. In the case of acetic acid and formic acid, these acids qualify as strongly corrosive when they encounter temperatures such as 90° C. to 250° C. in certain subteranian formations or used in mixtures with the other acids listed above.
The acids are selected based on well characteristics such as the tubular steel compositions and the geology of the rocks. The acids are mixed with corrosion inhibitors and other additives before they are injected into the wells. Some examples of these additives are iron control agents (e.g., citric acid, acetic acid), breaker materials (e.g., NaCl, CaCl2), scale inhibitors (sodium polycarboxylate, phosphonic acid salt), surfactants (nonionic, cationic and anionic), reducing agents (sodium erythorborate, thio compounds) and viscosity modifiers. All of the additive components should be selected so that they are mutually compatible when added to the acids.
The corrosion inhibitors/intensifiers of this invention may be combined with additional corrosion inhibitors (including conventional corrosion inhibitors) or corrosion inhibition intensifiers (CIIs). One aspect of this innovation is the use of solid corrosion inhibitor or CII components which have low water solubility. A highly preferred method of adding such materials according to the present invention involves preparing surface functionalized particles which can easily be dispersed in aqueous media. Low water solubility materials are defined as those which at room temperature have a water solubility of less than 100 mg/liter and preferably less than 15 mg/liter of water. The surface functionalization is typically carried out using materials which have a molecular weight of at least 60 and prefereably at least 80 and most preferably at least 100. More on surface functionalization and preparation of such particles is provided in published US patent application 2014/0271757 the disclosure of which is included herein by reference. Corrosion inhibitor formulations which combine several inhibitors synergistically is also an object of the present invention, as are corrosion inhibitors that reduce ferric ions to mitigate the corrosion caused by such ions and also to mitigate sludge formation caused by ferric ions.